Using an assay for G-actin based on its ability to inhibit the hydrolytic activity of DNAase, we have isolated a protein from brain which actively promotes the depolymerization of F-actin in solution. This protein will now be further characterized as to its molecular weight, isoelectric point, amino acid composition and its ability to interact with F-actin alone and in the presence of associated proteins such as tropomyosin. A modification of the DNAase inhibition assay has been developed which permits the assay of G-actin in the presence of F-actin under conditions where no exchange between the two actin pools occurs. This assay will be applied to the measurement of G and F-actin concentrations in synchronized CHO cells during the cell cycle.